A lead is a person or a company that has shown interest in your product or service.
Usually a lead fills out a lead form leaving basic business information including but not limited to name, last name, job title and the company they work for something in return, like an e-book for example.
There are three types of leads, based on their level of engagement with your brand and content:
Marketing Qualified Lead (MQL)
Sales Qualified Lead (SQL)
Product (Service) Qualified Lead (PQL / SQL)
Note: The pages for the specific leads can be linked to above